We would do it step by step:-
On the W3C markup validator, while validating the website, go to More Options => show Source and Show Outline, and validate the markup.
You now can scroll down and see Validation Output: 46 Errors
1. Give alt attribute to all the <img> tags
For example, <img src="images/testimage" alt="put image alternative text here" />
alt is the alternative text for the images, so that search engines could know what the image is for
2. Write the br tag as <br />, dont forget the slash
3. Maximum errors are for <embed> tag, I would explain it to you in next reply, once you fix the above small errors
This is because the markup errors propagate...

so let us see how many are reduced once you fix these